1. Field of the Invention
The present invention pertains to a method and apparatus for interfacing a plurality of Gas Measurement Systems to a Host System via an interface unit, and to a sidestream Gas Measurement System that is capable of communicating with a Host System that is specifically configured to communicate with only a mainstream Gas Measurement System.
2. Description of the Related Art
Respiratory Gas Monitoring Systems typically include gas sensing, measurement, processing, communication, and display functions. They are generally classified by operating modality as either diverting (i.e., sidestream) or non-diverting (i.e., mainstream). A diverting Gas Measurement System transports a portion of the sampled gases from the sampling site through a sampling tube to the Sample Cell, where the constituents of the gas are measured by a Gas Sensor. The sampling side is typically a in breathing circuit coupled to the patient's airway or at a location near the patient's airway. A non-diverting Gas Measurement System, on the other hand, does not transport gas away from the breathing circuit or the patient's airway, but measures the gas constituents passing through a Sample Cell disposed in the breathing circuit.
An example of a conventional mainstream Gas Measurement System 8 is schematically illustrated in FIG. 1. Mainstream Gas Measurement System 8 includes a Sample Cell 10 disposed in a breathing circuit 12 such that gas delivered to and/or received from the patient, as indicated by arrow A, passes through the Sample Cell. A Gas Sensor 14 coupled to the Sample Cell produces a detected or measured signal, e.g., a voltage, indicative of the concentration of a gas constituent in the gas sample in the Sample Cell. Gas Sensor 14 communicates with Sample Cell 10 placed at the breathing circuit and includes the components required to output a detected signal corresponding to a property of the gas to be measured.
For example, in a conventional mainstream Gas Measurement System that is capable of measuring carbon dioxide, Gas Sensor 14 includes a source 16 that emits infrared radiation, as indicated by arrow B, encompassing an absorption band for carbon dioxide. The infrared radiation is transmitted along a path that is normal to the flow path of the respiratory gas stream through the Sample Cell. Gas Sensor 14 in this conventional system further includes photodetectors 18 that measure the transmitted radiation. Carbon dioxide in the sample gas absorbs this radiation at some wavelengths and passes other wavelengths.
A multi-conductor lightweight, flexible cable 20 transmits the detected signals output by photodetectors 18 to a Gas Monitor 22 from which the partial pressure of carbon dioxide CO2 is calculated. In a conventional mainstream system, the Gas Monitor is a stand-alone unit contained within a housing 26 that includes a terminal 27 to which cable 20 is selectively coupled. Within housing 26, the Gas Monitor includes processing elements that convert the detected signal from the Gas Sensor into a value, such as transmittance, that is used to produce an indication of the concentration of a particular gas constituent in the gas sample within the Sample Cell. This value indicative of the concentration of the gas under analysis is provided to a Host System 24 also contained in housing 26, which uses this information in any one of a variety ways. For example, the Host System may display the indication of the designated gas as a waveform or as a value in partial pressure units, such as mmHg or concentration in units, such as a percent (%). The Host System can use the indication to calculate other parameters, which can then be displayed or communicated to another system, such as a central station.
In the present example, the calculated CO2 partial pressure is typically graphically displayed in the form of a capnogram via an output device serving as the Host System, such as a display provided on the exterior of the housing 26. Thus, Gas Monitor 22 contains the processing elements that control the operation of Gas Sensor 14 and provide the gas measurement functions to the Host System based on the output signals from the Gas Sensor. An example of such a conventional mainstream Gas Measurement System is shown in U.S. Pat. No. 4,914,720 issued to Knodle et al.
An advantage of a mainstream Gas Measurement System is that the placement of the Sample Cell directly at the breathing circuit results in a “crisp” gas concentration waveform that more faithfully reflects, in real-time, the varying partial pressure of the measured gas, such as carbon dioxide, within the airway that is generally possible using a sidestream approach. In addition, locating the Sample Cell, which is also referred to as a cuvette or airway adapter, in the respiratory gas stream obviates the need for gas sampling and scavenging as required in a sidestream Gas Measurement System.
As example of a conventional sidestream Gas Measurement System 28 is shown in FIG. 2. Sidestream Gas Measurement System 28 utilizes a long sampling tube 30 connected to an adapter 32 connected in line in breathing circuit, such as a T-piece connected at the endotracheal tube or mask connector. It is also known to use a nasal canula as the gas collection element in a conventional sidestream system so that the gas is taken directly at the patient's airway. The sample gas is continuously aspirated from the breathing circuit or the nasal canula through sampling tube 30, as indicated by arrow C, and into a Sample Cell 10′ at sample flow rates ranging from 50 to 250 ml/min. A pump 34 is typically provided to draw the gas into the Sample Cell from the gas sample site.
Sample Cell 10′ is contained in a housing 36, which also contains both a Gas Sensor, generally indicated at 38, and a Gas Monitor 22. As with a mainstream system, the Gas Sensor in the sidestream Gas Measurement System includes components that provide a detected signal indicative of the concentration of a gas constituent in the gas sample in the Sample Cell, such as source 16 and emitter photodetectors 18. Similarly, the Gas Monitor in the sidestream Gas Measurement System includes the processing elements that convert the detected signal from the Gas Sensor into a value used to produce an indication of the concentration of a particular gas constituent. This value is provided to Host System 24, where it is typically displayed on a display provided on housing 36. Examples of conventional sidestream Gas Measurement Systems are taught in U.S. Pat. No. 4,692,621 to Passaro et al.; U.S. Pat. No. 4,177,381 to McClatchie, and U.S. Pat. No. 5,282,473 to Braig et al.
A large installed base of mainstream Gas Measurement Systems exist. However, a growing number of mainstream Gas Measurement System users desire or require the use a sidestream Gas Measurement System. These users seek a simple and easy solution to add the sidestream gas measurement capability to their existing patient monitor inventory without having to replace completely or partially the existing mainstream Gas Measurement Systems. However, existing mainstream Gas Measurement Systems do not allow users to easily add the sidestream sampling function, because they are specifically configured for mainstream operation.
Some manufactures have addressed this dilemma by providing a gas monitoring system that includes both the mainstream and sidestream functionality in a single enclosure. Of course, this approach is relatively expensive. It is also known to use a mainstream Gas Measurement System in a sidestream-type gas scavenging system. For example, U.S. Pat. No. 4,958,075 to Mace et al. (“the '475 patent”) teaches providing a sidestream Gas Measurement System that includes most of the hardware required by a sidestream Gas Measurement System, such as a long tube, a Sample Cell disposed along the length of the tube, and a pump for drawing gas into the tube. However, instead of providing a Gas Sensor in the housing to which the Sample Cell attaches, as done in the conventional device shown in FIG. 1, the system taught by the '475 patent provides a mainstream type Gas Sensor coupled to the Sample Cell. In effect, the '475 patent teaches moving the Gas Sensor functions of a sidestream system out of the housing containing the Gas Monitor functions. It can be appreciated, that this approach still requires the use of a specialized adapter, which is connected to a pump, as well as valving, and control circuitry in the same box.
It is desirable to permit users of existing mainstream Gas Measurement Systems to add the sidestream gas measurement capability in a manner that permits their investment in mainstream Gas Measurement Systems to be preserved, while allowing addition of the sidestream gas measuring feature, as needed. Additionally, it is further desirable to permit users to not be burdened with having to employ complete systems, or a burdened mainstream Gas Measurement System, in order to gain the advantages of mainstream gas measurement, while permitting the use of sidestream gas measurement.